Many people buy a lot more and on a frequent basis. You should be just fine. If you want to play it safe, I suggest buying the gold on a mule account then trade it to yours. Which ever you decide, goodluck.
You should buy it if you don't really need the money for anything else, and the risk of getting banned is so low you don't even need to worry about it, just don't get scammed whilst buying it
i feel you wont get banned for something as insignificant as 15m, but keep in mind mod weath is a real bitch, like i dont like him. He actually takes his job serious compared to rs3 mods that search for cheaters,hes banned many of my friends for rwt major. Its at your risk, just keep in mind he takes his job serious.
I honesty Buy GP every week (30m+) and I havent got banned yet. Around 2 months and I bought a total of 320m+ So I don't believe you will, unless you have bad luck and jagex catches the gold seller.